1) Field of the Invention
The field of the invention pertains to communications and, more particularly, to antenna diversity in a wireless communication system.
2) Background
A common problem in wireless communications is interference caused by nearby transmissions on the same or adjacent frequency bands. Interference can cause a receiver to misinterpret a transmitted signal, become jammed, fail to correlate a received signal, or can have other adverse effects on the ability of a receiver to properly receive transmitted information. In addition, receivers in a wireless communication system can also suffer from multipath and fading, which can cause the received signal to fluctuate in amplitude, phase or other characteristics in a relatively short period of time (e.g., a matter of milliseconds).
These problems (i.e., interference, multipath and fading) can be exacerbated in a communication environment in which the transmitters and/or receivers are mobile, due in part to the unpredictability of positioning of the mobile transmitters and/or receivers with respect to the interfering or potentially interfering sources (which may themselves be mobile). Also, some of the above problems can be aggravated when signals are transmitted and received over high frequencies, due to the shorter wavelength(s) of the high frequency signals.
In an attempt to overcome the above problems, some wireless communication systems use multiple antennas at the receiver so as to improve the likelihood of detecting a good quality signal sent from the transmitter. Likewise, some systems employ multiple antennas at the transmitter to increase the likelihood that one of the antennas will yield a superior communication path to the receiver. A variety of techniques are known in the art for selecting one or more antennas for use at a receiver or a transmitter when multiple antennas are available.
When multiple antennas are used at a transmitter or receiver, the antennas are often spatially separated, so that physical conditions responsible for causing fading, multipath or interference have less likelihood of affecting all of the antennas simultaneously. An antenna selection algorithm typically selects the best antenna or antennas for communication according to predetermined criteria such as received signal strength.
In some time division duplex (TDD) or time division multiple access (TDMA) communication environments wherein transmitters and receivers communicate only periodically, antenna selection can in certain instances be difficult due to the "bursty" nature of the periodic transmissions between communication points. In one type of multi-user TDD environment, a base station is generally capable of communicating with a different user station in each time slot. Antenna selection in this environment can be difficult because the antenna (or antennas) best suited for one user station in a first time slot will not necessarily be the most suitable antenna (or antennas) for a different user station, or even the same user station, communicating in a different time slot. Another problem is that the time available is limited for analyzing received signals and deciding which antenna of the multiple antennas is best. Consequently, if antenna selection is performed in a TDMA, TDD or bursty communication environment, it should be performed rapidly.
Rapid antenna selection can prove to be difficult if gaps occurring between received signals are relatively long, because the communication environment is more likely to change with the lengthening of the delay period between received signals. Mobile stations are particularly subject to a rapidly changing communication environment, because unpredictable physical obstacles or obstructions can create interference or cause a change in multipath characteristics, and because external interference sources can be encountered on an unpredictable basis. Moreover, in mobile systems the distance between the user station and the base station can change thereby causing a possible change in the received signal strength. Thus, a mobile communication environment can complicate an antenna selection process, and the relatively bursty nature of certain TDD or TDMA communication environments adds to the difficulty of selecting an antenna by providing only a relatively short amount of time for determining which antenna is the best to use.
While using multiple antennas can, as noted, lead to potentially improved communication quality in some circumstances, multiple antennas are likely to increase the size and/or bulkiness of a communication unit. In many applications, including wireless and/or mobile applications, it is generally desirable to provide communication with small, lightweight handsets. However, if multiple antennas are needed in the handset, the space necessary for the additional antennas can adversely impact the size and/or weight of the handset. While using only a single antenna at the handset can keep the handset smaller and lighter, conventional antenna diversity techniques generally require multiple antennas at the handset in order to achieve the benefits of antenna diversity at the handset.
There is a need for an antenna selection system that is particularly adapted to TDD, TDMA or bursty communication environments, and that can suitably respond to changing environmental factors which may affect the nature and/or quality of signals being transmitted and received. There further is a need for an antenna selection system that can handle multiple users communicating in different time slots, and select an appropriate antenna (or antennas) rapidly and on a slot-by-slot basis. There is also a need for an antenna selection system that minimizes the number of antennas needed at the user station or handset, while nonetheless providing antenna diversity benefits at the user station.